The present invention relates generally to the formation of structure in which titanium-alloy sheet material forms a component part. More particularly the invention relates to improved methods for forming sheets for incorporation as components into multiple sheet structures such as homeycomb structures and reinforced sheet structures and the like.
Titanium base alloys have been identified as potential matrix materials for use in metal matrix composites. Such composites may be formed, for example, by layering up multiple thin layers of the titanium with other materials. They have also been identified as potential ingredients for filament reinforced composite structures. Further they have been identified as components for super plastically formed diffusion bonded homeycomb structures.
The subject application relates generally to the first two copending applications referenced above in relevant subject matter. The methods described above relate to the formation of sheet articles of titanium by plasma deposition of titanium alloy onto a receiving surface which may be a rotating drum. Various uses are then made of the foil which is formed on the receiving surface in forming composite articles or in receiving reinforcement as with silicon carbide fibers to fabricate high performance structures.
I have found that the preferred powder size for use in the deposition of titanium alloys to form the sheet or foil products as referred to above is about 100 to about 250 .mu.m or greater and that it is preferred to deposit such powder using a RF powered plasma gun. When a foil is prepared with a structure formed from such larger size particles the as-sprayed surface roughness of the RF plasma sprayed foils can be quite high. The surface roughness can result in poor packing efficiency of titanium-alloy foils and SiC fibers.
Further such roughness can possibly lead to damage of the filaments which are to be embedded therebetween in pressurization steps or during HIPing or hot pressing consolidation processes. From work done with regard to such foils and thin sheet materials it has become evident that it is desirable to reduce the surface roughness in order to improve the usefulness of the foil formed by the plasma spray deposit process.
Novel and unique structures are formed pursuant to the present invention by plasma spray deposition of titanium base alloys and titanium-aluminum intermetallic compounds employing RF plasma spray apparatus and by then modifying the as-sprayed deposit.
The formation of plasma spray deposits of titanium base alloys, including intermetallic compounds of titanium, present a set of processing problems which are unlike those of most other high temperature high strength materials such as the superalloys. A superalloy such as a nickel base or iron base superalloy can be subdivided to relatively small size particles of -400 mesh (about 37 .mu.m) or smaller without causing the powder to accumulate a significant surface deposit of oxygen. A nickel base superalloy in powder form having particle size of less than -400 mesh will typically have from about 200 to about 400 parts per million of oxygen. A powdered titanium base alloy by contrast will typically have a ten fold higher concentration of oxygen. A powdered titanium base alloy of -400 mesh will have between about 2000 and 4000 ppm of oxygen.
Moreover titanium base alloy powder of less than -400 mesh size is recognized as being potentially pyrophoric and as requiring special handling to avoid pyrophoric behavior.
It is also recognized that the low temperature ductility of titanium base alloys decreases as the concentration of oxygen and of nitrogen which they contain increases. It is accordingly important to keep the oxygen and nitrogen content of titanium base alloys at a minimum.
Prior art plasma spray technology is based primarily on use of direct current plasma guns. It has been recognized that most as-sprayed plasma spray deposits of the superalloys such as nickel and iron base superalloys have had relatively low ductility and that such deposits when in their sheet form can be cracked when bent through a sufficiently acute angle due to the low ductility. Such plasma spray deposits do acquire improved properties based on heat treatment. However in the as-sprayed form they do have very limited ductility and are subject to cracking as noted.
I have discovered that RF plasma apparatus is capable of spraying powder of much larger particle size than the conventional d.c. plasma apparatus. I have discovered that particle sizes at least three times larger in diameter than those conventionally employed in d.c. plasma spray apparatus may be successfully employed as plasma spray practices and that the particle size may be as high as 100 .mu.m to 250 .mu.m and larger and as large as 10X as large as the -400 mesh powder previously employed in d.c. plasma spray practice.
This possiblity of employing the larger powder particles is quite important and for metal powders such as titanium base alloys which are subject to reactions and absorption of gases such as nitrogen and oxygen on their surfaces. One reason is that the surface area of particles relative to their mass decreases inversely as their diameters. Accordingly a three fold increase in particle diameter translates into a three fold decrease in particle suface area to volume. I have discovered that one result is that RF plasma spray deposited structures of titanium base alloys made with the aid of larger particles have lower oxygen content than might be expected based on knowledge of prior art practices.
As used herein the term titanium base alloys means an alloy composition in which titanium is at least half of the composition in parts by weight when the various alloy constituents are specified as in percentage by weight.
A titanium-aluminum intermetallic compound is a titanium base alloy composition in which titanium and aluminum are present in a simple numerical atomic ratio and the titanium and aluminum are distributed in the composition in a crystal form which corresponds to the simle numerical ratio such as 3:1 for Ti.sub.3 Al, 1:1 for TiAl; and 1:3 for TiAl.sub.3.
Ti.sub.3 Al compositions have use temperatures of up to about 1400.degree. F. as compared to the use temperature of titanium alloys such as Ti-6Al-4V of up to about 1000.degree. F. The use temperature of TiAl is in the 1700.degree.-1800.degree. F. range.